WordPress: When a thumbnail image is requested of a specific width/height (rather than by name), generate it if it doesn't exist.

generate-thumbnails.php

<?php /*

**************************************************************************

Plugin Name:  Generate Thumbnails On The Fly
Description:  When a thumbnail image is requested of a specific width/height (rather than by name), generate it if it doesn't exist.
Version:      1.0.0
Author:       Alex Mills (Viper007Bond)
Author URI:   http://www.viper007bond.com/

**************************************************************************/

add_filter( 'image_downsize', 'viper007bond_generate_thumbnails_on_the_fly', 10, 3 );

function viper007bond_generate_thumbnails_on_the_fly( $existing_data, $attachment_id, $size ) {

	// Let WordPress handle named thumbnail sizes
	if ( ! is_array( $size ) )
		return $existing_data;

	// Safety
	$size = array_map( 'absint', $size );

	// Get the path to the fullsize image
	$fullsize_path = get_attached_file( $attachment_id );

	// Split it into parts
	$fullsize_info = pathinfo( $fullsize_path );

	// Create the thumbnail filename
	// Is there a helper function for all of this? I couldn't find one.
	$thumbnail_filename = str_replace( ".{$fullsize_info['extension']}", "-{$size[0]}x{$size[1]}.{$fullsize_info['extension']}", $fullsize_info['basename'] );

	$thumbnail_path = $fullsize_info['dirname'] . '/' . $thumbnail_filename;

	// If the thumbnail already exists
	if ( file_exists( $thumbnail_path ) ) {

		// Create the URL to the thumbnail by taking the fullsize image
		// and replacing it's filename with the thumbnail filename
		$thumbnail_url = str_replace( $fullsize_info['basename'], $thumbnail_filename, wp_get_attachment_url( $attachment_id ) );
	}

	// Okay, thumbnail doesn't exist. Make it!
	else {

		// Have to crop so that width/height is exact and findable in the future.
		$new_thumbnail_path = image_resize( $fullsize_path, $size[0], $size[1], true );

		if ( is_wp_error( $new_thumbnail_path ) )
			return $existing_data;

		// Get the thumbnail path parts, specifically the filename
		// Yeah, we created it above but let's be absolutely sure it's correct
		$new_thumbnail_info = pathinfo( $new_thumbnail_path );

		$thumbnail_url = str_replace( $fullsize_info['basename'], $new_thumbnail_info['basename'], wp_get_attachment_url( $attachment_id ) );
	}

	return array(
		$thumbnail_url, // URL
		$size[0],       // Width
		$size[1],       // Height
		true,           // is_intermediate, i.e. exact size or will it be resized via HTML?
	);
}

A lot of this has changed with newer versions of WordPress, I forked a modified version here using the WP_Image_Editor class: https://gist.github.com/davegaeddert/bf02b6745eca850556a9